The electrically erasable programmable read only memory (EEPROM) cells with coupler regions are widely used in analog and mixed signal applications. The EEPROM memory cell generally comprises a floating gate, an oxide layer and a coupler region within which a coupler channel region is defined under the floating gate. The coupler channel region typically comprises an inversion layer and a depletion layer.
During an erase operation, an erase voltage is applied in the coupler region and is then capacitively coupled to the floating gate with a coupling ratio which is partly determined by the width of the depletion layer. In more detail, the wider the depletion layer is, the smaller the coupling ratio is. Take a P-type coupler region for example, the depletion layer is formed by removing the mobile holes and leaving the fixed ions in the space charge region. The depletion width expands until the inversion layer is formed by attracting the mobile electrons of the coupler region at the interface between the coupler region and the oxide layer. However, as the mobile electrons are minority carriers in the P-type coupler region, the formation of the inversion layer is slow, causing a wide depletion layer which, in turn, results in a small coupling ratio. Therefore, the time to erase the EEPROM memory cell is long, and the absolute value of the erase voltage is large. And in addition, the EEPROM memory cell may have a bad linearity of the final erase threshold voltage to the erase voltage for a short erase time.
Thus, an EEPROM memory cell having a large and time independent coupling ratio between the coupler region and the floating gate is desired.